Reflow ovens have come a long way, they are much more stable, better controlled, well built and less expensive. Nevertheless, the need for profiling has not been eliminated, since after all the reflow oven is just a means to an end. You still need to dial in your oven set points to achieve a desired profile and what oven set point temperatures you choose is not what you see on your PCB. You may be surprised how many line operators worldwide do not make this connection!

Once you have developed numerous profile recipes, many of you would only like to spot check the reflow oven to see if there has been any change. The logic goes, if I have developed robust profiles and I can check my oven periodically to confirm there hasn’t been any change, I can therefore infer that my profiles are still valid? Well, yes and no. “Infer” is the action word here that can lead to trouble if you are not careful. Let me give you an example.

For fun I ran some popular oven verification tools you can find on the market to see if they could catch changes that I knew would throw my profiles out-of-spec. What I did first was to profile an actual PCB, followed by my oven verification fixture. Running the fixture soon after my profiles gives me a baseline to compare against for future spot checks. I then took a small clip on desk fan and blew into the entrance of my oven tunnel, knowing that it would change the thermal dynamics inside my process. I made sure my fan was low enough not to trigger an oven alarm, but just enough airflow that based on my experience would in fact change the process environment inside my oven.

Here is what I found. My profiled board width was ~12″ wide so a sizable mass. I used a fixture of the same width, since changing my belt width is not practical. When I ran the fixture I was given oven statistics to compare against my future spot check. No problem so far. When I ran the fixture a second time (fan a blowing), I found a 1 degree delta change. Now I think you would agree 1 degree doesn’t seem like much at all. Of course I am not looking at a profile, rather machine data so here is that word, I “inferred” that my process was still fine. Wouldn’t you? Now intuitively, I knew this fan blowing up the tunnel of my oven is creating process changes perhaps more than 1 degree. So how do you know, well you run a profile! Sure enough when I ran a profile, I did not see any longer just a 1 degree delta. In fact my profile was out of spec.

So what happen? I can only assume the fixture which I was using that weighed in at close to 7 lbs, must be absorbing some of the changes inside the oven, masking what is really going on. Keep in mind the fixture is not designed to profile, rather it is designed to detect measurable change to my process that would impact my profile. The mass itself must be factoring into my ability to detect change. Again this is just my observations and my conclusions are subjective, but I can say that it did not capture what is the whole point of this exercise. So I guess the mass of your vehicle is important and it is not all just relative. Any thoughts out there, please post your comments.

Well I guess it is back to profiling, or is it? Stay tuned for part II as I explore other options.

Well if you have ever used Kapton tape to attach a thermocouple, you have certainly seen your share of profiles like this!

So what, it is a perfectly good profile, right? Yes, but no. I had a customer who was using KIC’s Navigation (auto prediction) to help create a better “deep in-spec” profile. The only problem, they were trying to optimize on a TC reading that was bouncing literally all over their PCB. Navigator is an awesome tool, but it can only work with what you feed it. If you feed it garbage, it will give you garbage. In their case, it was trying to find them a new solution where literally every time the board was run the bouncing TC that was attached (or I should say was not very well attached) with Kapton was giving false readings. Navigator would give a different solution based on what the TC was reading at that given time. It is like try to put post-it notes on the ocean.

Solution is very simple, eliminate the TC reading from your graph. You can easily do this with the profile you just ran. Look what happens, you go from a far out-of-spec of 126% PWI to a far in-spec of 48% PWI.

So you saved your hard work this time, but you are after all one thermocouple reading short. You added that TC to your profile for a reason. Next go around, do yourself a favor and use a better material for attachment, such a conductive double side aluminum tape, which by the way, a recent study from RIT proves it a superior attachment method aside from sticking to your PCB much better.

I leave it to the screen printer, pick and place and reflow oven guys to answer the equipment part of the equation, but I can answer how one can determine with a profile if your BGA is getting what it needs as well as how other aspects of your PCB are impacted.

Across the Belt Uniformity:

There can be anywhere from a 2 – 5+C variation in temperature across the belt. For example, BTU uses this homemade fixture to test for uniformity. The idea is fairly simple. With a set of type K calibrated thermocouples, you can easily monitor 6 TCs across the belt. You want obviously to see the least amount of variation (if you were wondering the front TC is for measuring air temperature which is also used for automatic mapping of the profile to the oven zones with KIC2000 software).

Profiling for Reflow:

BGAs typically require more heat to reach their peak temperatures than smaller massed components like electrolytic capacitors. For example, your BGA might have a peak temperature of 245C.

While your electrolytic capacitors cannot tolerate as high as a peak temperature, therefore you want to set their maximum peak temperature lower, for example to 235C (this is just a relative example).

With KIC2000 software, you can define each component in isolation. If the BGA is off on the edge, I might need to bump up even further my peak temperature spec since in many reflow ovens, the outer edge near the rail is the coolest. This is why you see some ovens with heat tape running along the rails! Keep in mind of course as you crank up your oven to reach higher temps to reflow your outer edge BGAs, everything else on your board is also going to be impacted. More the reason you better be hooking up thermocouples to temperature sensitive components to ensure they do not get fried while you are focusing your attention on your BGAs. Profiling software that can “balance” the board is a must. If there ever was a case where software can help solve complex problems in profiling, here you go!

The Rochester Institute of Technology under the guidance of Dr. S. Manian Ramkumar Ph.D. just conducted (October 2009) the most comprehensive study to date on thermocouple attachment methods. Part I of II was to determine the most accurate and reliable method of thermocouple attachment. Part II that has yet to be released is to determine the best attachment methods for BGAs, with the goal of seeing if there are reliable non-destructive methodologies, so stay tuned.

Results in a nutshell:

Aluminum Tape out performed all materials even Kapton! In an ideal word, the best attachment method of a thermocouple to a component is what I like to call a naked TC. Aluminum double sided conductive tape was the closest thing to having nothing at all to attach the thermocouple. Kapton tape is less responsive (deflecting and insulating heat), never mind if you have ever seen a saw-tooth TC plotted on a profile you know it has a very hard time staying in place on your PCB. Additionally, High Temperature Solder which I have always considered the gold standard, is the least accurate or responsive. When you get to the critical peak temperature of your profile, high temperature solder is sluggish to respond to the rapid change in temperatures, thus distorting your readings. As Phil Zarrow and Jim Hall discuss in Board Talk, “mass” on your thermocouple is not your friend. Phil Zarrow:

any measurement method, the key element is to get the thermocouple in good contact with what you are trying to measure and to do it in a way that does not modify the area with a lot of extra mass or material that is going to give you an inaccurate reading….

Bingo! This is actually what this study shows, now with the numbers to back it up.

Three identical test coupons were used and run multiple times. KIC’s air-TC was utilized as the control to which each thermocouple was measured as the coupon traveled through all heated zones.

A total of three boards were used, running each board through twice, allowing the internal temperature of the KIC device to drop below 40 degrees C before rerunning the profile.

The tape attach methods were measured uniformly for each RTD connection, using a dial caliper, while the high temperature solder and epoxy quantities for attach were found to be visually uniform.

This graph indicates the mean temperature differential that was noticed within the oven for the various attach methods. The readings are based upon the complete profile starting at room temperature and ending at the peak temperature. The data from the cool down zone was eliminated from the analysis.

The graph shows the mean differential and the 95% confidence interval for each attach method. The Aluminum tape had the least differential (-0.48) followed by Kapton Tape, Loctite Adhesive, CW-2400 and then HT-Solder. The Confidence intervals among most of the attach methods do not overlap except Kapton and Loctite, indicating that the means of the attach methods are significant. Significant differences exist between the methods except between Kapton and Loctite as there is overlap. Clearly Aluminum tape outperforms all of the other methods.

The thermocouples seem to behave similarly within each of the zones of the oven. Zone 6, where the soldering takes place or the peak temperature is reached, the thermocouple attach methods show a much higher temperature than the air temperature, indicating that the PCBs have attained much higher temperatures than the air. A closer examination of ZONE 6 reinforces the selection of Loctite or Aluminum Tape for Phase III of this project.

Conclusion:

When considering accuracy, repeatability and responsiveness, Aluminum Tape is a winner. There are of course advantages and disadvantages to each material. For example one can argue you can re profile a PCB set up with high temperature solder, but considering that the mass of the solder distorts your readings, this study even brings into question this bedrock of thermocouple attachment. Never mind high temp solder destroys your PCB as well as there is little control over the size of the blog from TC to TC and board to board. Also don’t forget every time you profile the same board again it loses some mass, which will be the focus of more blogs to come.